1. Field of the Invention
The present invention relates to a display technology for a liquid crystal display (LCD), in particular, to a method and apparatus for increasing the number of gray levels in a LCD.
2. Background Art
Today, the term “liquid crystal display (LCD)” may readily bring a color LCD display to mind. In fact, most LCD modules widely used for LCD monitors use an “8-bit color” source driver, which displays each of the colors, read (R), green (G), and blue (B), by using 8-bit data. By this technology, 28=256 gray levels of each color can be represented, and therefore totally as much as (28)3=16 M (approximately 16 million) colors can be represented by all of the R, G, and B.
On the other hand, applications of displays do not necessarily require color display capability. For some applications, a monochrome display is sufficient, or even, better than a color display to meet the demands for higher resolution and a larger number of gray levels. A good example is medical image displays used for x-ray. CRT monitors capable of displaying monochromes with higher resolution and more gray levels have been used conventionally in these special applications. Some monochrome CRT monitors can display 12-bit data, that is, data that can represent 212 gray levels, supplied by the graphics adapter of a host system. To support such data, LCD displays are required to be able to represent that number of gray levels.
The market of these monochrome monitors is very attractive to the manufacturers of LCD modules/monitors. Today, LCD monitors can provide very high resolution, such as Quad Extended Graphics Array (QXGA: 2048×1536 dots) resolution and Quad Ultra Extended Graphics Array (QUXGA: 3200×2400 dots) resolution. Some LCD monitors even surpass CRT monitors in pixel pitches. For example, the pixel pitches of a 20.8-inch LCD monitor having QXGA are as follows:in row: (4/5).20.8.25.4/2048=0.20637in column: (3/5).20.8.25.4/1536=0.20637
Thus, the pitches are approximately 206 μm in both of the horizontal and vertical directions. This is too fine to the human eye for displaying characters (a pixel pitch of approximately 300 μm is said to be suitable for displaying characters), but best suited for displaying graphics.
As described above, the resolution of the LCD monitors is adequately high. However, the number of gray levels, which can be displayed by the LCD monitors, is very poor. For example, the number of gray levels displayable on a monochrome TFT monitor depends on the number of bits converted by the X-driver (digital-analog converter) of a LCD monitor. A monochrome TFT monitor using an 8-bit digital-analog converter can represent only 256 levels. Therefore, smoothly varying gray levels in a natural image are not always achieved. In particular, the number of gray levels of the LCD monitors is inadequate for applications, which require true gray scale images, such as the above-mentioned medical images (such as x-ray images).
Here, consider the case where a color Thin Film Transistor (TFT) LCD panel is made monochrome by simply removing its color filters (for example, by eliminating the color filter generation process). In that case, the number of gray levels representable by one pixel can be increased by treating three pixels corresponding to R, G, B as one monochrome pixel and combining the gray levels of these sub-pixels. In the case an 8-bit color image is made monochrome, when the gray level value of the three sub-pixels is changing from (m, m, m) to (m+1, m+1, m+1) (where, 0≦m≦28−1), the sub-pixels can take two brightness levels, from (m, m, m+1) to (m, m+1, m+1). Here, (m, m, m+1), (m, m+1, m), and (m+1, m, m) are considered as the same brightness level and cannot be distinguished from each other. The same applies to (m, m+1, m+1), (m+1, m, m+1), and (m+1, m+1, m). Therefore, the number of representable gray levels is 3.(28)−2=766.
This will be described below in detail. The gamma (γ: applied voltage (gray level) versus the transmittance of liquid crystal (brightness) of the sub-pixels, which is set by the X-driver of the LCD, can be changed by changing the reference voltage provided to the X-driver, which is a digital-analog converter. However, the gamma of each sub-pixel set by the X-driver cannot be individually changed because of limitations of the driver. Thus, the gammas of the sub-pixels will be the same. Here, assuming that the brightness of areas which was called R, G, B is N. Then, the gray levels of each of the R, G, B areas can be expressed as 0, N/255, 2N/255, . . . , 255N/255. By combining R, G, and B, gray levels of 0, N/255, 2N/255, . . . , 765N/255 can be represented. Thus, even if the color filter is removed from the color LCD panel and one pixel is represented by three sub-pixels to display monochrome images, the number of gray levels provided by an 8-bit color display device is at most 766, which is below 210. Therefore, this approach cannot significantly increase the number of displayable gray levels.
The present invention is made in order to solve the above-mentioned problems and it is an object of the invention to increase the number of gray levels displayable on an LCD display without applying any optical arrangements such as filters to the surface of the LCD or increasing the number of bits provided by the X-driver (for example, eight bits) of the current LCD display.
Another object of the present invention is to allow an existing X-driver to be shared among sub-pixels without making any special changes to the gamma of the X-driver in order to increase the number of gray levels.